The present invention relates in general to low physical profile antennas and, in particular, to a coplanar microstrip antenna having three resonances. The invention relates especially to a microstrip antenna having three-frequency operation in which two frequencies can be spaced slightly apart to achieve a circularly polarized signal at the midfrequency point and an elliptically polarized signal at the third frequency.
One design of multiple-frequency antennas employs an antenna structure in which single-band microstrip radiating elements are stacked above a ground plane with the surface of each element dimensioned so as to resonate at a different frequency. Each of the radiating elements is fed with a separate feedline, either a coplanar feedline or a coaxial-to-microstrip adapter normal to the plane of the radiating element. The multiple layers and the multiple feedlines result in a less compact and more complex structure than is desirable for some aerospace applications.
Multiple band operation has been provided using microstrip antennas and feed networks etched on the same surface. U.S. Pat. No. 4,356,492 discloses a dual band antenna in which two single-band coplanar radiating elements are fed from a common coplanar input point.
Instantaneous dual band operation using single element microstrip antennas and feed networks etched on the same surface require either (1) microstrip antennas with a single feedline on the same surface as the antenna (coplanar antenna) or (2) diplexed output ports on the feed network. In general, dual band, coplanar, single feedline antenna designs are available only if the frequencies of interest are within 15 percent of each other or are harmonically related. Diplexers in the feed network result in a larger, less efficient, and more complex microstrip antenna array.
Copending U.S. patent application, Ser. No. 856,569, now U.S. Pat. No. 4,692,769, entitled Dual Band Slotted Microstrip Antenna, by the same inventor as in the present application, discloses instantaneous dual band operation in a slotted microstrip radiating element wherein the two resonances are perpendicularly polarized and may be separated by as much as a 2:1 ratio.
However, none of these foregoing designs provides instantaneous triple frequency operation of a microstrip antenna and an undiplexed feed network etched on the same surface. Nor do these designs provide instantaneous dual frequency operation of a microstrip antenna and undiplexed feed network in which one frequency is circularly polarized and the other frequency is linearly polarized. Instantaneous triple frequency operation or dual frequency operation in which one frequency is circularly polarized and the other elliptically polarized allows a smaller, more efficient and less complex microstrip array antenna.